Facsimile and static presentation processing – Facsimile – Specific signal processing circuitry
Patent
1991-12-19
1993-04-27
Britton, Howard W.
Facsimile and static presentation processing
Facsimile
Specific signal processing circuitry
H04N 7137
Patent
active
052067238
DESCRIPTION:
BRIEF SUMMARY
The present invention concerns motion estimation, particularly, though not exclusively, in the context of video coders employing inter-frame differential coding.
FIG. 1 shows a known form of video coder. Video signals (commonly in digital form) are received at an input a. A subtractor b forms the difference between the input and a predicted signal from a predictor c, which difference is then further coded in box d. The coding performed here is not material to the present invention, but may include thresholding (to suppress transmission of zero or minor differences) quantisation or transform coding, for example. The input to the predictor is the sum, formed in an adder e, of the prediction and the coded difference signal decoded in a local decoder f (so that loss of information in the coding and decoding process is included in the predictor loop).
The differential coding is essentially inter-frame, and the predictor c could simply consist of a one-frame delay; as shown, however, a motion estimator g is also included. This compares the frame of the picture being coded with the previous frame being supplied to the predictor. For each block of the current frame (into which the picture is regarded as divided), it identifies that region of the previous frame which the block most closely resembles. The vector difference in position between the identified region and the block in question is termed a motion vector (since it usually represents motion of an object within the scene depicted by the television picture) and is applied to the predictor to shift the identified region of the previous frame into the position of the relevant block in the current frame, thereby making the predictor output a better prediction. This results in the differences formed by the subtractor b being, on average, smaller, and permits the coder d to encode the picture using a lower bit rate than would otherwise be the case.
One type of motion estimator works on a block by block basis by comparing each block with the corresponding block of the previous frame and regions positionally shifted from that block position; this involves a considerable amount of processing and often necessitates many accesses to stored versions of both frames.
The present invention is defined in the claims.
Embodiments of the invention will now be described, with reference to the accompanying drawings, in which:
FIG. 1 shows a known form of video coder;
FIG. 2 shows a block of the present picture and a corresponding search area of the previous picture;
FIG. 3 shows schematically a motion vector estimator according to a first embodiment of the invention;
FIG. 4 shows schematically a Previous Picture Array according to one embodiment of the invention;
FIG. 5 shows schematically a search scan produced by the invention;
FIG. 6 shows a second embodiment of the invention;
FIG. 7 shows schematically the blocks of the present picture processed by the embodiment of FIG. 6;
FIG. 8 shows schematically an arithmetic unit, suitable for calculating a similarity measure as part of the invention; and
FIG. 9 shows schematically a minimum similarity measure store suitable as part of the invention.
The motion estimator to be described regards a "current" frame of a television picture which is being coded as being divided into 8.times.8 blocks--that is, eight picture elements (pixels) horizontally by eight lines vertically. Although the principles are equally applicable to interlaced systems, for simplicity of description a non-interlaced picture is assumed. It is designed to generate for each block a motion vector which indicates the position of the block-sized region, lying within a defined search area of the (or a) previous frame of the picture, which is most similar to the block in question.
FIG. 2 illustrates a field with an m.times.n=8.times.8 block N (shaded) and an associated 23.times.23 (i.e. 8+7.times.2) pixel search area indicated by a rectangle S.sub.N. If the pixels horizontally and lines vertically are identified by coordinates x, y, with an origin at the top left-hand
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IEEE/Teile Globecom Conference 87, Tokyo, Japan, Nov. 15-18, 1987, vol. 1, IEEE. T. Russell Hsing: "Motion detection and compensation coding for motion video coders: technical review and comparison"--pp. 2.5.1-2.6.4.
Proceedings of the International Conference on Acoustics, Speech, and Signal processing, Dallas, Texas, Apr. 6-9, 1987, vol. 2, IEEE A. Puri et al: "An efficient blockmatching algorithm for motion-compensated coding" pp. 25.4.1-25.4.4.
British Telecommunications public limited company
Britton Howard W.
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